1. Field of the Invention
The present invention relates to an average value calculating circuit, and a correlation value calculating circuit, a matched filter and a communication device using the same.
2. Description of the Related Art
FIG. 8 is a schematic block diagram showing a prior art receiver for direct sequence (DS) spread spectrum communication.
A signal received by antenna 7 selectively passes through band-pass filter 8 and is provided to detecting circuit 9. Detecting circuit 9 performs envelope or synchronous detection of the signal modulated according to ASK, FSK or PSK and converts it into analog or digital spread spectrum signal DIN. Matched filter 10P calculates correlation value DOUT between spread spectrum signal DIN and a pseudorandom noise (PN). Determining circuit 11 determines the symbol duration for acquisition based on correlation value DOUT and obtains baseband data. To reduce the amount of data, if the baseband data are coded, for example, according to predictive coding, the baseband data are decoded by decoding circuit 12. In the case of sound data, the output of decoding circuit 12 is converted into an analog value by digital-to-analog converter circuit 13, and then, passes through low-pass filter 14 and is provided to speaker 15. In the case of data such as image or text data, the output of the decoding circuit 12 is used as reproduction data.
The spread spectrum communication using such a receiver is superior to other communication schemes in interference wave excluding capability, concealability of the contents of communication and frequency use efficiency.
FIG. 9 shows the construction of conventional matched filter 10P.
In digital or analog shift register 20, delay elements DL1 to DLn are cascaded. To the data input of delay element DL1, spread spectrum signal DIN is provided. In synchronization with clock CLK, input signals DIN and S1 to Sn-i of delay elements DL1 to DLn are held therein and they output delay signals S1 to Sn. Delay elements DL1 to DLn are flip-flops if spread spectrum signal DIN is digital, and are sample-and-hold circuits or CCDs, etc. if the spread spectrum signal DIN is analog.
Coincidence degrees D1 to Dn between pseudorandom noise P1 to Pn and delay signals S1 to Sn are calculated by coincidence degree calculating circuits M1 to Mn, respectively. Coincidence degree calculating circuits M1 to Mn are, for example, multipliers if pseudorandom noise is 1 or -1, and are exclusive NOR gates if pseudorandom noise is a bit of `1` or `0.` Coincidence degrees D1 to Dn are provided to adder circuit 21 and the sum total thereof is obtained as correlation value DOUT.
With such matched filter 10P, correlation value DOUT is immediately obtained every clock period.
Pseudorandom noises vary from receiver to receiver. When the pseudorandom noise on the receiving side differ from that on the transmitting side, correlation value DOUT is always low, so that the received data cannot be decoded.
For example, for n=256, it is necessary to calculate the sum total of coincidence degrees D1 to D256 in one period of clock CLK. Consequently, construction of adder circuit 21, the correlation value calculating circuit, the matched filter and the communication device is complicated.